The present invention relates to a CDMA telecommunications system. More specifically, the present invention relates to a CDMA telecommunication system which employs an adaptive rate power control system.
Code Division Multiple Access (CDMA) telecommunications systems are well known and include, for example, cellular telephone systems constructed in accordance with the IS-95 standard. As is well known, the performance of such systems depends critically upon the ability of the system to control the transmission power at both the base station and the mobile stations. Transmissions from a mobile unit in a cell act as interference to the transmissions of each other mobile unit within the cell. For multiple access to be achieved in an efficient manner, the signal of each mobile user should be received at the base station at the minimum power level for acceptable reception. Without power control, the signal of a mobile unit adjacent a base station would typically be received at a much higher power level than that of a mobile user at the reception fringe of the base station and this is typically referred to as the near far problem in CDMA systems. Similarly, the transmissions of a base station to mobile units within a cell act as interference to the transmissions of base stations in adjacent cells. Thus, controlling the transmission power within a CDMA system is an important factor in obtaining efficient use of the bandwidth and resources of the system. Two power control techniques are employed with IS-95 systems: (1) Open Loop power control; and (2) Closed Loop power control.
In open loop power control, the mobile unit measures the level at which a transmission signal from the base station is received and compares that level to the level the base station transmitted the signal at (which is known) to derive an estimate of the path signal loss, The mobile unit then adjusts it""s own transmission level appropriately based on the assumption that its transmitted signal will experience the same path loss to the base station.
Closed loop power control comprises the base station determining the actual received signal strength from each mobile unit and instructing each mobile unit to increase or decrease its transmission power accordingly. In IS-95 systems, to compensate for fast fading experienced by mobile units as they move about, frequency separation of uplink and downlink channels, etc., this power adjustment is performed eight hundred times a second and the power control signals are inserted into the data channel from the base station to the mobile unit by puncturing the signal transmitted to the mobile unit to include the power control signals.
While IS-95 systems and the like have worked reasonably well with mobile units, it is an expensive power control system, both in terms of the computational complexity required to implement the power control, especially the closed loop power control, and the amount of bandwidth required to send the closed loop power control signals. In the latter case, the puncturing reduces the effective data rate between the base station and the mobile unit and/or increases the bit error probability for the signal.
It is therefore desired to have a power control method and system which does not suffer from these disadvantages.
It is an object of the present invention to provide a novel method and system for power control in a CDMA telecommunication system which obviates or mitigates at least some of the above-identified disadvantages of the prior art.
According to a first aspect of the present invention, there is provided a method of adaptively setting a rate at which power control information is exchanged between a base station and a subscriber station, comprising the steps of:
(i) said subscriber station reporting to said base station an indication of the level at which it has received a transmission from said base station;
(ii) said base station receiving said indicated levels and determining the amount of variation between a set of said indicated levels;
(iii) if said amount of variation exceeds a first predefined value, said base station instructing said subscriber station to increase the rate at which it performs step (i) and recommencing the method at step (i);
(iv) if said amount of variation is less than a second predefined value, said base station instructing said subscriber station to decrease the rate at which it performs step (i) and recommencing the method at step (i); and
(v) if said amount of variation is less than said first predefined value and more than said second predefined value, recommencing the method at step (i).
According to another aspect of the present invention, there is provided a method of adaptively setting a rate at which power control information is exchanged between a base station and a subscriber station, comprising the steps of:
(i) said subscriber station reporting to said base station an indication of the level at which it has received a transmission from said base station;
(ii) said base station receiving said indicated levels and determining the amount of variation between a set of said indicated levels;
(iii) if said amount of variation exceeds a first predefined value, said base station instructing said subscriber station to increase the rate at which it performs step (i) and recommencing the method at step (i);
(iv) if said amount of variation is less than a second predefined value, said base station instructing said subscriber station to decrease the rate at which it performs step (i) and recommencing the method at step (i); and
(v) if said amount of variation is less than said first predefined value and more than said second predefined value, recommencing the method at step (i).
The present invention provides a novel power control system for CDMA telecommunications systems wherein the rate of power updates can be changed, adaptively, as needed and up to date power control information is available, via a user control channel, to establish a dedicated channel between stations in an efficient manner.